This invention relates to housings for electrical elements. More specifically, the invention provides an improved housing for process electronics associated with a measuring or sensing device, such as a differential pressure transmitter.
Differential pressure transmitters are commonly used in process control systems that require pressure, flowrate, or measurements of other variables associated with gases and liquids. One typical prior differential pressure transmitter employs a two-section housing assembly. A lower section of the transmitter assembly communicates with an upper section of the transmitter assembly by way of a port extending between the two sections.
The lower section typically houses, among other components, two process diaphragms and a transducer. The lower section includes two process fluid ports for coupling to input process lines for exposing each process diaphragm to one of two fluid pressures that are to be compared. Each process diaphragm deflects in response to,the pressure of one fluid. The transducer responds to the difference between the two pressures of the process fluids, and produces electrical output signals for indication or control.
The upper section typically houses process electronics, i.e. electronic circuits and electrical components for monitoring and conditioning the electrical output signals from the transducer for transmission to a display meter, and/or for application to a control valve, a computer or another electronic device. In the case where the conditioned signals are to be applied to a computer or other electronic device, they can be coupled out of the second section of the housing assembly, by way of fittings to electrical conduits.
Typical process fluids can be corrosive and/or toxic. Accordingly, the upper section of the housing assembly, which contains the process electronics, is isolated from the process fluids that are coupled to the lower section of the housing assembly. Since the process fluids are only applied to one side of each process diaphragm, the two process diaphragms and lower section of the housing assembly form a chamber that is isolated from the process fluids and is generally filled with an inert fill fluid. Additionally, this chamber leads to a transducer for generating the electrical output signals to the process electronics.
One disadvantage of some prior fluid measuring systems of the foregoing type is that if either process diaphragm fails, the process fluid can displace the inert fill fluid and the corrosives or volatiles may eventually vent throughout the upper section of the housing assembly, destroying the process electronics. Additionally, the process fluids can then vent from the second section through the electrical conduits, by way of the fittings in the housing assembly. This can be especially dangerous if the process fluids are toxic or flammable. Some prior housing assemblies address this problem by providing a barrier wall that divides the second section into two compartments; one containing the port for communicating between the first and second sections of the housing assembly and the other containing the fittings that couple to the electrical conduits.
Nevertheless, these prior systems continue to suffer from several drawbacks. For example, some employ hard-wired electrical penetrators to feed signals through the barrier wall, and consequently suffer from reduced flexibility with regard to allocating circuitry between the two compartments. Others require that an opening through the barrier wall in the second section be either custom cast or custom machined. Still others have reduced access to the circuitry housed in the second section.
Another drawback to some prior systems is that the two chambers that are created by the barrier wall are not environmentally controlled. Since one chamber is in communication with the lower housing section by way of the port, the process electronics and part of the lower housing section are exposed to any humidity present in the chamber. Hence, some prior devices require all of the electronic components in the upper section of the housing assembly to be sealed or coated from ambient moisture, typically by being potted or conformally coated with a moisture barrier. This can be expensive and can complicate access to the components during maintenance and repair.
In most differential pressure transmitters, the upper section of the housing assembly is accessible at two axial ends by way of threaded covers. While these assemblies provide access to the upper section of the housing assembly, they may have significant disadvantages. One such disadvantage arises because the prior art assemblies typically provide only two compartments in the upper section. Thus, if a cover is removed, all of the electronics housed in that compartment are exposed to the outside environment. Hence, operation in a humid environment can, once again, require the circuits contained in the upper section to be sealed from moisture. Another disadvantage is that such covers allow unrestricted access to the electronics. Thus, the instrument can be inadvertently damaged or subjected to tampering.
It is accordingly an object of this invention to provide an improved housing assembly for process electronics that precludes process fluids from venting into the electrical conduits, upon a failure of one or both process diaphragms.
Another object is to provide a housing assembly that provides improved flexibility with regard to arranging process electronics in the housing, while still ensuring that process fluids are blocked from the electrical conduits.
It is also an object of the invention to provide an improved housing assembly that can contain various types of process electronics, without modifications to the housing.
A further object of the invention is to provide a housing assembly that has improved access, and yet is tamper-resistant.
An additional object of the invention is to provide a housing assembly that minimizes environmentally infiltration while eliminating the need for seal-coating the process electronics, by potting or otherwise.
Yet another object of the invention is to provide a housing structure that has relatively few parts and is relatively economical to manufacture, and that can house a variety of processing electronics, for varied commercial applications.
Other objects of the invention will in part be obvious and will in part appear hereinafter.